This invention relates generally to fire alarm systems, and more particularly, to methods and apparatus for verifying power conditions at notification appliances during low voltage situations.
Notification appliances are typically installed as part of fire alarm systems. During the installation process, the appliances need to be verified to ensure operation under all designated circumstances. Under normal operating conditions, an AC branch circuit provides a primary source of power to a control panel. This is the condition under which the system is typically checked for proper operation. Under this condition, the notification appliances are likely to have adequate operating voltage and will operate properly.
Fire alarm systems typically have a secondary source of power, such as storage batteries. Fire alarm codes, such as NFPA 72, require that the system be operable for a minimum period of time when using the secondary power source, such as 24 hours, 60 hours or other length of time specified by the Authority Having Jurisdiction (AHJ).
As the batteries are discharged, the output voltage supplied to the notification appliances decreases. Therefore, the system is required, such as by Underwriter's Laboratories, to operate with the power source at 85% of the rated input voltage. For example, a fire alarm system may utilize 24V batteries as standby power sources. In this case, the system is specified to be fully operational when the battery voltage is reduced to 20.4V. The intent of the codes and standards is that the system will operate for the specified standby period after which the system must operate in the alarm condition. The alarm condition is the most severe load condition for the system.
The wiring to all alarm devices and appliances is to be verified upon installation to ensure the input voltage and current limitations for each notification appliance remain within the specified range for operation. Many of the notification appliances in use are “constant power” loads. Therefore, when input voltage is reduced, the current increases, and the current draw of a notification appliance at reduced voltage is higher than when the input voltage is at the normal operating voltage. The increase in current draw at lower voltages also results in greater line loss than when operating under normal conditions. When the system is verified during installation, the wiring distance may be verified to ensure that the wiring voltage loss to each notification appliance does not reduce the input voltage to any notification appliance on the circuit to below the rated input voltage.
Notification appliances may be wired as notification circuits or as signaling lines. When wired as notification circuits, the wiring is routed from the control panel to each device in succession. When wired as signaling lines, the wires may spoke off to form multiple wiring runs, each of which has a different wire resistance that is unknown to any degree of accuracy.
Installation verification methods vary, but overall are time-consuming, expensive, and often inadequate and prone to error when testing actual low input voltage conditions. In addition, the labor required to properly test the system is expensive, and schedule and/or financial pressure could cause an installer to forego a complete and accurate verification. For example, operating the system at normal input voltage and observing all notification appliances for proper operation does not verify that the system will operate properly at low input voltage. The voltage may be manually measured at each appliance, which verifies adequate voltage under normal operating conditions, but does not confirm the voltage level under a low voltage condition. The worst-case voltage drop for each wiring run may be calculated based on low-battery operation, but this method often results in severely limiting wiring distance, which is undesirable.
In addition, the line losses are difficult to estimate as the current varies across the entire length of the circuit. As stated previously, line loss increases with lower input voltage. Thus, if the voltage is measured at a remote notification appliance under normal operating conditions, calculating the worst-case condition by determining the present line loss and subtracting it from the low input voltage is not accurate.
Alternatively, the system may be operated from the secondary (battery) source for the specified standby period. At the end of the standby period, the system is operated in the alarm state and the notification appliances are verified. This method is very costly, time consuming and potentially disruptive. In addition, it is difficult to precisely discharge the batteries, and an over-discharge condition can permanently damage the batteries.
Therefore, a need exists for a method and apparatus for verifying the operation of notification appliances during a low input voltage condition. Certain embodiments of the present invention are intended to meet these needs and other objectives that will become apparent from the description and drawings set forth below.